The invention relates to a stabilized conductive pigment based on aluminum-doped zinc oxide (AZO). In many areas of industry, there is a need for conductive pigments which make it possible to produce, for example, plastics, paints, coatings or fibers which are electrically conductive, antistatic or provide shielding from electromagnetic waves. For this purpose, conductive carbon black is used in large amounts but cannot be employed for transparent, light-colored or colored coatings due to its high light absorption in the visible region of the spectrum. Another problem is the strong absorption of carbon black in the IR region, as a result of which the coated articles heat up when exposed to solar radiation for example, which is often undesirable. In the case of light-colored coatings, carbon black has therefore increasingly been replaced by antimony-doped tin oxide.
Another conductive pigment which is suitable for light-colored coatings is aluminum-doped zinc oxide.
DE-A 4,023,802 describes the preparation of needle-shaped zinc carbonate, which is used as a precursor for producing needle-shaped electrically conductive zinc oxide. When doped with aluminum, indium, gallium and tin, the products obtained are light grey to light brown. Doping with germanium produces a white powder. The electrically conducting zinc oxide powder needle-shaped particles having a length ratio of 3 to 400 or platelet-like particles having a length/thickness ratio of 10 to 1000. Calcining of the pigment is carried out in a reducing atmosphere under nitrogen.
EP-A-0,404,087 and EP-A-0,405,364 disclose a needle-shaped electrically conducting zinc oxide and a process for its preparation.
An alkali metal zincate solution containing a water-soluble compound of at least one of the metals tin, gallium, indium and aluminum is neutralized with an inorganic acid in such a manner that the pH at the end of the reaction is in the range from 7 to 12. The resulting mixed precipitation products are filtered off, washed and dried and then calcined in a reducing atmosphere. The electrically conducting zinc oxide obtained contains 0.005 to 5 parts by weight of the metal oxides used as doping substance per 100 parts by weight of zinc oxide. Compared with incorporated spherical electrically conducting zinc oxide, the electrical volume resistivity of a film containing the needle-shaped product is lower by about 3 powers of ten. The needle-shaped structure of the zinc oxide leads to better conductivity of the pigmented film.
Furthermore, JP-A-03/200,877 discloses an electrically conducting powder consisting of a support and a zinc oxide layer deposited thereupon. The zinc oxide is doped with a tri- or tetravalent metal. This metal can be aluminum, germanium, gallium, tin or indium.
Examples of the supports used are mica, kaolin, zinc oxide, titanium oxide, glass fiber or glass platelets. The electrically conductive powder is prepared by suspending the support in the aqueous solution of a zinc salt of an organic acid and of a salt of a tri- or tetravalent metal, adjusting the pH to 5 to 9, and depositing the precipitated metal hydroxides/hydrated metal oxides on the support.
The resulting product is separated off, washed and dried and calcined at 500.degree. to 1300.degree. C., preferably 500.degree. to 900.degree. C., in a reducing gas atmosphere.
Conductive pigments based on zinc oxide have the disadvantage that storage of these pigments in air leads to a marked decrease in their electric conductivity over time. The resistivity, which is about 2 to 4 ohm.times.cm immediately after preparation of the pigment, can rise to several times the original value in the course of a few weeks.
This is why it has been suggested in EP-A-0,500,445 to protect the conductive layer against oxidation by overcoating it with a metal oxide, for example alumina or silica.
However, a protective layer made of a non-conducting material has the disadvantage of reducing the conductivity of the pigment.